1. Field of the Invention
This invention relates to a narrow directional microphone comprising an acoustic tube, and more particularly relates to a narrow directional microphone of which directivity is not lowered even if water droplets sticks onto the acoustic tube.
2. Description of the Related Art
Usually, a narrow directional microphone comprises a long thin acoustic tube. A microphone unit is attached in a rear end of the acoustic tube, detects acoustic waves arriving via a front end of the acoustic tube, and converts the acoustic waves into audio signals.
With an example of the narrow directional microphones of the related art, an acoustic tube has slits on an inner peripheral surface thereof. The slits are covered by an acoustic resistor made of a thin synthetic resin film or non-woven fabric. Such a narrow directional microphone makes acoustic waves arriving via the front end thereof interfere with acoustic waves arriving from the peripheral surface via the acoustic resistor and the slits, thereby assuring the directionality.
The directionality of such a narrow directional microphone varies with a changing acoustic resistance of the acoustic resistor extending over the slits on the acoustic tube. For instance, when the narrow directional microphone is used outdoors, raindrops may stick onto and moisten the acoustic resistor. In such a case, the acoustic resistance will be increased, acoustic waves do not pass through the acoustic resistor, and the directivity of the microphone will be lowered.
The assignee of this application has already proposed a narrow directional microphone in which slits on the acoustic tube are covered by a thin synthetic resin film or non-woven fabric in order to improve the directivity. Refer to Japanese Laid-Open Patent Publication No. Sho 62-118,697 (called the “Reference 1.). However, when an acoustic resistor is moistened, the acoustic resistance will be increased or acoustic waves do not pass, which means that the directivity of the microphone will be lowered.
One example of existing narrow directional microphones will be described hereinafter. Referring to FIG. 4(A) to FIG. 4(C) and FIG. 5(A) to FIG. 5(C) of the accompanying drawings, a narrow directional microphone 10 includes an acoustic tube 20, slits 22, an acoustic resistor 24, a microphone unit 30, and a cylindrical grip 40. The acoustic tube 20 is a thin long cylinder made of metal, and has the slits 22 extending along the center axis thereof and on an inner peripheral surface thereof. The slits 22 are arranged at equal intervals. The acoustic resistor 24 extends over the inner surface of the acoustic tube 20 and the slits 22. The acoustic tube 20 has open ends. In those figures, each left end of the acoustic tube 20 is a front end while each right end thereof is rear end. The microphone unit 30 is provided in the rear end of the acoustic tube 20. The cylindrical grip 40 is coupled to the rear end of the acoustic tube 20.
Acoustic waves arriving via the front end of the acoustic tube 20 and those arriving via the slits 22 and the acoustic resistor 24 are made to interfere with one another, which promotes the narrow directivity. Refer to FIG. 4(A) to FIG. 4(C) and FIG. 5(A) and FIG. 5(C).
Further, the narrow directional microphone using the acoustic tube suffers from noises caused by winds. The rear end of the acoustic tube is connected to a front acoustic terminal of the microphone unit. When the acoustic tube is coupled to the microphone unit, a distance is extensively increased between the front and rear acoustic terminals, which would result in noises caused by winds. The assignee of this application has also proposed a narrow directional microphone which includes a microphone unit and measures against noises caused by winds. Refer to Japanese Patent Laid-Open Publication No. Hei 11-331,978 (called the “Reference 2”). In the Reference 2, a clearance is provided between an outer peripheral surface of a microphone unit and an inner peripheral surface of an acoustic tube, and front and rear acoustic terminals of the microphone unit are short-circuited by acoustic impedance in the foregoing clearance. Acoustic waves having very low frequencies such as noise caused by winds can be short-circuited by the acoustic impedance, which is effective in reducing noise caused by winds.
Noises caused by vibrations in the narrow directional microphone depend upon mass of air in the acoustic tube. Therefore, the longer the acoustic tube, the more the mass of air in the acoustic tube and the more noises caused by vibrations. The invention in the Reference 2 can also reduce the noises caused by vibrations since the front and rear acoustic terminals of the microphone units are acoustically short-circuited.
As described above, the Reference 2 is effective in reducing noises caused by winds and vibrations. The narrow directional characteristics of the Reference 2 are improved by making acoustic waves arriving via the front end of the acoustic tube and the slits on the peripheral surface of the acoustic tube interfere with acoustic waves arriving via the slits and the acoustic resistor. Therefore, if the acoustic resistor is moistened by raindrops, the directivity of the microphone will be damaged. In other words, the microphone is protected against noises, but no measures are taken in order to prevent the directivity from being lowered by moisture.